A piezoelectric motor uses a piezoelectric vibrator to transduce electrical energy into kinetic energy that the motor transmits to a moveable body to which the motor is coupled. The motor is generally coupled to a body that it moves by resiliently pressing the motor to the body so that a surface region, hereinafter referred to as a “motor-coupling-surface”, of its piezoelectric vibrator contacts a surface, hereinafter referred to as a “body-coupling-surface”, of the body. Electrodes comprised in the motor are electrified (generally with an AC voltage) to excite vibrations in the vibrator that cause the motor-coupling-surface to vibrate. Motion is transmitted from the vibrating motor-coupling-surface to move the body by frictional forces between the motor-coupling-surface and the body-coupling-surface.
In general, two orthogonal resonant vibration modes of a piezoelectric vibrator are simultaneously excited to generate vibrations in the motor-coupling-surface that are suitable for transmitting motion to the moveable element. A first resonant vibration mode, hereinafter referred to as a “perpendicular vibration mode”, moves the motor-coupling-surface back and forth in a direction perpendicular to the body-coupling-surface. A second resonant vibration mode, hereinafter referred to as a “parallel vibration mode”, of the vibrator moves the motor-coupling-surface back and forth parallel to the body-coupling-surface. As a result of the perpendicular back and forth motion of the motor-coupling-surface generated by the perpendicular resonant vibration, the two coupling surfaces are alternately coupled and uncoupled during each vibration cycle of the perpendicular vibration mode. During times when the motor-coupling surface and body-coupling-surfaces are in contact, motion of the motor-coupling-surface parallel to the body-coupling-surface generated by the parallel resonant vibration mode transmits motion to the body.
To efficiently transmit motion to a moveable body, the perpendicular and parallel vibration modes of a piezoelectric motor must generally have excitation curves as a function of frequency that overlap substantially. As a result of the overlap, electrodes in the piezoelectric motor can be electrified to excite the vibration modes with an AC driving voltage having a frequency, hereinafter referred to as a “driving frequency”, at which energy is simultaneously coupled efficiently to both vibration modes. In addition, when excited, the perpendicular and parallel vibration modes should optimally have a phase difference, hereinafter referred to as a “mode phase difference”, close to 90°. If the mode phase difference is substantially different from 90°, the perpendicular and parallel vibration modes are not properly synchronized and efficiency with which motion of the motor-coupling-surface transmits motion to the body is reduced.
For a piezoelectric vibrator, density and Young's modulus of the piezoelectric material from which the vibrator is formed and dimensions of the vibrator determine resonant frequencies of vibration modes of the vibrator. For a given piezoelectric material, characterized by a given Young's modulus and density, the dimensions of the vibrator determine the resonant frequencies. In addition the dimensions determine other operational characteristics of the vibrator. For example, the amplitude of motion of a motor-coupling-surface of the vibrator and the amount of power that the vibrator can provide for moving a body are functions of dimensions of the vibrator as well as the magnitude of an applied excitation voltage. Maximum strain in the body of the vibrator, for a given excitation voltage, is also a function of dimensions of the vibrator.
As a result of the many operational parameters of a piezoelectric vibrator that are functions of the vibrator's dimensions, it is generally not possible to determine dimensions of a vibrator that optimize all operational characteristics of the vibrator. In particular, it is not always possible or practical to determine dimensions of a vibrator so that excitation curves of a perpendicular and a parallel vibration mode overlap at a frequency at which both vibration modes can be efficiently excited with a 90° mode phase difference. It is therefore advantageous to have a method for shifting resonant frequencies of a vibrator without having to substantially change dimensions of the vibrator. Such a method could be useable to shift resonant frequencies of a perpendicular and parallel vibration mode of a vibrator to adjust overlap of their excitation curves and improve efficiency with which the vibration modes can be simultaneously excited with a mode phase difference close to 90°.
In some situations, instead of requiring two orthogonal vibration modes to impart motion to a moveable body, a piezoelectric motor is required to provide one-dimensional motion of a motor-coupling-surface. For such motion, optimum efficiency of operation of the motor is generally achieved when only a single vibration mode is excited in the motor's vibrator. For a vibrator having dimensions that provide some desired characteristics of the required motion more than one vibration mode may be excited at a driving frequency at which the motor is to be operated. In such cases it is desirable to have a method for diverging resonant frequencies of vibration modes that are simultaneously excited without substantially changing dimensions of the vibrator so as to separate their excitation curves and enable excitation of substantially only the single vibration mode of the vibrator.
PCT Application PCT/IL99/00576 entitled “Piezoelectric Motors And Motor Driving Configurations”, the disclosure of which is incorporated herein by reference, describes situations for which excitation of a single vibration mode is advantageous. In the application, shavers are described that comprise a piezoelectric motor that is used to excite vibrations in cutting blades comprised in the shavers. A one-dimensional motion generated in a motor-coupling-surface of the piezoelectric motor is used to excite the vibrations.
Another situation for which it is sometimes desirable to diverge resonant vibration modes of a piezoelectric vibrator occurs, for example, when it is desired to separately control perpendicular and parallel vibration modes of a piezoelectric vibrator. Optimum control of the vibration modes is obtained when the excitation curves of the vibration modes are sufficiently separated so that energy can be coupled to either one of the vibration modes without substantially coupling energy to the other of the vibration modes. Examples of situations for which it is advantageous to excite perpendicular and parallel vibration modes of a vibrator separately, and methods for exciting the vibration modes, are described in PCT Application PCT/IL99/00288, entitled “Multilayer Piezoelectric Motor”, the disclosure of which is incorporated herein by reference.
It is also desirable to be able to shift a resonant frequency of a vibration mode of a vibrator in order to reduce a difference that the frequency may have from a desired frequency that is caused by inaccuracies in the process by which the vibrator is manufactured. For example, assume that in a manufacturing process of a vibrator in the shape of a relatively thin rectangular plate having large face surfaces and narrow long and short edge surfaces, the length of the vibrator is held to a tolerance of 1%. Assume further that a desired resonant frequency of a vibration mode of the vibrator for which mass points of the vibrator vibrate parallel to the length of the vibrator is, by way of example 50 kHz. Since the length of the vibrator may vary by as much as 1%, the resonant frequency, which is proportional to the inverse of the length, may vary by as much as 0.5 kHz from 50 kHz. It is desirable to be able to shift the resonant frequency after manufacture of the vibrator to compensate for the variance.
Thin rectangular piezoelectric vibrators are described in U.S. Pat. No. 5,616,980 to Zumeris, the disclosure of which is incorporated herein by reference. Various methods are used to mount these motors in machines and apparatus in which they are used. One of the mounting methods used, which is described in the patent comprises forming mounting holes that pass through the body of the vibrator in directions perpendicular to the vibrator's large face surfaces. The vibrator is held in place in the machine or apparatus by mounting pins anchored to the machine or apparatus that pass through the mounting holes. Spaces between the mounting holes and the mounting pins are preferably filled with a flexible material. As described in the patent, characteristics of the filler material are preferably chosen to match the acoustic velocity of the filler to the acoustic velocity of the piezoelectric material from which the vibrator is formed. Matching the acoustic velocities reduces the amount by which the holes and the pins disturb resonant frequencies of the vibrator.